The present invention is generally related to telecommunications switching, and more particularly to mapping of a multicast circuit identifier to at least one local circuit identifier in an Asynchronous Transfer Mode ("ATM") switch.
ATM switches typically include a plurality of ingress cards, each card having a plurality of ingress ports and supporting a plurality of connections or circuits, a plurality of egress cards having a plurality of egress ports, and a switch fabric disposed between the ingress and egress cards. ATM cells are received at an ingress card, transmitted through the switch fabric to an egress card, and then transmitted from the egress card to a point downstream from the switch. The switch fabric interconnects each ingress card with each egress card such that a cell received at any one of the ingress cards can be transmitted to any of the egress cards.
In multicast operation a single cell received at one ingress card is transmitted to a plurality of downstream connections. Such operation may require transmission from a plurality of egress cards. Further, each egress card may include a plurality of ports, and the multicast cell may be transmitted through any number of such ports.
Destination address information located in a header portion of the multicast cell is modified for each copy of the cell to be transmitted in a multicast connection. The incoming cell has a Virtual Path Identifier and Virtual Circuit Identifier ("VPI/VCI") which are operated upon to provide a Multicast Circuit Identifier ("MID") portion which is written to the cell header. Copies of the cell are then generated, and each cell copy is tagged at the egress card with a locally unique Circuit Identifier ("CID") portion that becomes part of the cell header. The CID provides identification of the egress circuit and egress port associated with the cell. In practice, each MID is mapped to multiple CIDs. The CID then functions as an index into a translation table that contains outgoing VPI/VCI fields that are written to the cell header to facilitate downstream routing of the cell.
Blocks of contiguous entries are allocated in a table in memory at the egress card to implement mapping for support of multicast transmission. In particular, one contiguous block of entries is allocated for each multicast connection, including allocation of one entry for each port in the egress card. The entries are contiguous for the full number of ports associated with the egress card. However, not all multicast connections utilize all of the ports in each designated egress card. Hence, memory space that is allocated to such ports is not being efficiently utilized. Further, modification of a unicast connection to a multicast connection is inhibited in the event that contiguous memory locations have been previously allocated to other connections.